1. Technical Field
The present invention relates to antenna systems in general, and in particular to an antenna system adapted to a relatively low frequency band of wireless wide-area network.
2. Description of Related Art
A laptop personal computer (laptop PC) includes many antennas mounted thereon for handling different wireless communications systems such as WiMAX, wireless local-area network (wireless LAN), and wireless wide-area network (wireless WAN). A laptop PC performs data communication through the wireless WAN established by using a mobile phone communications network. In North America, primarily, the third generation (3G) personal communications service (PCS) band and the cellular band are available as the mobile phone frequency bands. The PCS uses the 1,900 MHz band. The cellular band has been the 850 MHz band. In Europe, primarily, the GSM 900/1,800 MHz band and the UMTS 2,100 MHz band have been used as the mobile phone frequency bands.
Further, in the 700 MHz band, a fourth generation (4G) mobile communications service based on the communications standard called Long-Term Evolution (LTE) has been started. In the United States, Verizon Wireless Inc. offers the LTE service using the 750 MHz band (from 747 MHz to 787 MHz), and AT&T Inc. offers the LTE service using the 700 MHz band (from 704 MHz to 746 MHz). Further, in Europe, Vodafone Inc. is planning to offer the LTE service using the 790 MHz band (from 790 MHz to 862 MHz).
An antenna increases in length and size as the resonance frequency decreases. Further, the antenna gain decreases when a sufficient element length cannot be secured for the resonance frequency. In the case of adopting the LTE using the 700 MHz band, the required element length further increases. In a laptop PC, an antenna is disposed inside the rim of the display casing so as to obtain good radio properties during the use. Inside the rim of the display casing, a camera, a microphone, and an LED for illuminating the keyboard surface are disposed in addition to the antenna. Thus, a problem has arisen that, with the space conventionally available for the wireless WAN antenna, it would be difficult to guarantee sufficient gain for the frequencies near 700 MHz.
Meanwhile, a circuit board on which a camera and a microphone are mounted may be destroyed by a surge current that flows in from the outside through an opening of the display casing due to electrostatic discharge (ESD). Therefore, a countermeasure against ESD has been taken for the circuit board. Specifically, the ESD countermeasure for the circuit board is implemented by covering the part of the circuit board that is vulnerable to ESD, with a conductive sheet serving as an arrester.
The conductive sheet is connected to a ground plane of a motherboard via a shield of a signal line connected to the circuit board. A conductor that is maintained at the ground potential existent in the vicinity of the antenna may adversely affect the radio properties of the antenna.
Consequently, it would be desirable to provide an antenna system that can be disposed in a narrow space in a wireless terminal device such that the antenna can be placed as far apart as possible from the shielded line or conductive material connected to the conductive sheet.